Retaspimycin is a potent and water-soluble Hsp90 inhibitor(EC50s of 119 nM for both Hsp90 and Grp9).

Grp94:119 nM （EC50）, HSP90:119 nM （EC50）

Retaspimycin is a potent Hsp90 inhibitor(EC50s of 119 nM for both Hsp90 and Grp9). Retaspimycin is cytocoxic to human multiple myeloma (MM) cell lines(EC50s of 307 ± 51 nM and 306 ± 38 nM, respectively, for MM1.s and RPMI-8226 cells)[1]. Retaspimycin (IPI-504, 10-100 nM) suppresses the growth of both trastuzumab-sensitive and -resistant cells in a dose-dependent manner. Retaspimycin (0-500 nM) decreases HER2 protein expression and suppresses both Akt and MAPKs pathways in both sensitive and trastuzumab-resistant cells[3].

selective tumor retention in RPMI-8226 tumor-bearing mice caused by Retaspimycin. The tumor volume by 69% and and 84% of baseline values in GIST-882 and GIST-PSW xenografts reduced by Retaspimycin, respectively. Furthermore, Retaspimycin in combination with imatinib inhibits tumor growth more significantly than Retaspimycin alone in GIST-PSW model, but no obvious difference is ovsrebed in the GIST-882 model. Retaspimycin also downregulates KIT in gastrointestinal stromal tumor (GIST)[2]